Some people have posted packs in this comments section, there's also an unofficial Discord server for the game somewhere in these comments. Vinny/Joel are probably also sent packs directly by their moderators.
Can also back this, "Simpsons voice pack" is seen in Vinny's latest Choicer Voicer video: Vinny - The Choicer Voicer (Early Access Version 0.4.13) (youtube.com), so his moderators must be sending the packs to Vinny and Joel.